The present invention relates to a novel image reader for use in an apparatus for reading images and effecting information processing thereof, such as facsimile equipment or printers, and more particularly to a reader including, as a photoconductive element, a uniform thin film of amorphous silicon that is sandwiched between electrodes.
Semiconductor image sensors comprising charge-coupled devices (CCD) or photodiode arrays such as MOS photodiode arrays have heretofore been widely used as readers in facsimile equipment or printers. Since the known image sensors ordinarily incorporate an optical system for reducing the size of images, they may include lenses which contribute to a lengthy light path, making it difficult to achieve a small-size device.
There has recently been proposed a contact-type thin-film reader dimensioned to have the same width as that of a manuscript to be read thereby. The proposed reader is known as a planar-type reader which, as shown in FIG. 1 of the accompanying drawings, comprises an insulating substrate 1, a photoconductive layer 2 deposited thereon, and a pair of electrodes 3, 4 mounted on the photoconductive layer 2, the device thus constructed corresponding to one bit. A plurality of such devices are arranged in an array, and photoelectric currents generated by these devices upon being illuminated with light are successively detected for reading information.
With the proposed device, it is impossible to bring the electrodes 3, 4 closer than a distance of about 10.mu. to one another, and hence the speed of response, that is, the available reading speed, is limited. For example, a time required for scanning one line on an A4 sheet is 20 msec. with a resolution of 8 elements/mm. Such a limited reading speed is too low to meet the reading requirement needed by facsimile apparatus or printers that find higher-speed applications.
The photoconductive element used in the known reader is formed of chalcogenide glass, for example, such as Se-As-Te, which will be crystallized at around 80 degrees Celsius and hence is unable to operate stably for a long period of time. Where compounds are used, it is difficult to control the amounts of components during the manufacturing process because of the complex composition, with the result that fabricated photosensitive devices have characteristics which vary from device to device.
Various devices have been developed since the published articles relating to pn control of amorphous silicon through impurity doping, that is, "Substitutional Doping of Amorphous Silicon" by W. E. Spear et al., Solid State Communications, Vol. 17, pp. 1193-1196, 1975, and "Electronic Properties of Substitutionally Doped Amorphous Si and Ge", Philosophical Magazine, Vol. 33, No. 6, pp. 935-949, 1976. For example, U.S. Pat. No. 4,064,521 to Carlson discloses a solar cell utilizing photoconductive characteristics of amorphous silicon. Efforts to develop photosensitive devices are shown in Japanese Laid-Open Patent Publication Nos. 54-150995, 55-39404, and 56-2784. The photosensitive devices incorporating amorphous silicon are advantageous in that they are more resistant to heat than those of chalcogenide glass, immune to crystallization, and can easily be pn-controlled.
Conventional photosensitive devices of amorphous silicon include semiconductive junctions such as p-i-n, p-n, or Schottky barrier junctions, and are subject to unstable photoconductive characteristics due to difficulty encountered in the control of film formation while fabricating layers of amorphous silicon. Such devices also require insulation between bit-forming photoconductive elements of amorphous silicon to prevent an inter-bit current leakage due to a low resistance of 10.sup.4 -10.sup.6 .OMEGA.cm of an n-type layer out of these amorphous silicon layers. The known photosensitive devices also cannot meet the demand for an elongate structure or high-speed response to applied light, because of the foregoing problems experienced with the manufacturing process.